Gold is typically recovered from ores, concentrates and scraps using conventional cyanidation technology. In the tank leaching process, the slurry pH is adjusted to between 9.5 and 11 with lime, and cyanide is added to solubilize the gold. Air or oxygen is introduced to the slurry by injection or agitation, as applicable. Gold dissolves by the following reaction:4Au+O2+8CN−+2H2O→4Au(CN)2−+4OH−  (1)
Gold can be recovered from cyanide solution by adsorption onto activated carbon particles, either during the cyanide leach itself via carbon-in-leach (CIL) or following the leach by carbon-in-pulp (CIP). An alternate method of recovering gold from cyanide leach solutions is through zinc cementation and variations of the Merrill-Crowe process. Aurocyanide can also be recovered from pregnant solutions by ion exchange resins from which gold is eluted with thiourea.
Ball et al., U.S. Pat. No. 4,902,345, disclose treating refractory carbonaceous and sulfidic ores by thiourea leaching in the presence of carbon rather than cyanide leaching.
Marchbank et al., U.S. Pat. No. 5,536,297 discloses recovering gold from refractory carbonaceous ores by pressure oxidation and thiosulfate leaching. Ji et. al, U.S. Pat. No. 6,660,059 discloses an additional method for thiosulfate leaching using sodium thiosulfate.
Thiosulfate leaching of gold is a potentially attractive alternative to the conventional cyanidation process for at least three types of gold ore feed material. First, in gold ores that contain organic carbonaceous material, gold recovery by thiosulfate leaching is usually significantly higher because the gold thiosulfate complex is less sensitive to preg robbing. Secondly, gold/copper ores are frequently not well suited to the cyanidation process owing to higher cyanide consumption by the copper mineralization in the ore. In this case, thiosulfate leaching may be more economical due to the lower reagent consumption, as thiosulfate does not react as readily with copper. Finally, there are certain gold ore bodies that cannot be treated by the conventional cyanidation process due to local environmental restrictions.
The thiosulfate leach process has been proven technically viable, and many aspects of the process are disclosed in publications and patents. For example, Berezowsky et al., U.S. Pat. No. 4,070,182, disclose a process to leach gold from copper-bearing sulfidic material with ammonium thiosulfate, followed by cementation of the gold on zinc dust. Kerley Jr., U.S. Pat. Nos. 4,269,622 and 4,369,061, disclose using an ammonium thiosulfate leach solution containing copper to leach gold and silver from ores containing manganese. Perez et al., U.S. Pat. No. 4,654,078, disclose leaching gold and silver with a copper-ammonium thiosulfate lixiviant to produce a pregnant leach solution, from which gold and silver are recovered by copper cementation. Wan et al., U.S. Pat. No. 5,354,359, disclose leaching gold from preg robbing ores with a thiosulfate lixiviant followed by cementation or precipitation of the leached precious metal values. PCT application WO 91/11539 discloses recovering gold from a gold-loaded thiosulfate solution by adding cyanide to form a gold cyanide complex followed by adsorbing the gold cyanide complex onto a carbon or resin adsorbent. Thomas et al., U.S. Pat. Nos. 5,536,297 and 5,785,736, disclose a process for treating a refractory sulfidic and carbonaceous ore by pressure oxidation followed by thiosulfate leaching and adsorption of the gold thiosulfate complex on an ion exchange resin.
Thiosulfate may be less economically attractive than cyanidation because its leaching kinetics often demand higher reagent concentrations, and because reagent losses occur when thiosulfate is oxidized to polythionates or sulfates. Use of lower reagent strengths (U.S. Pat. No. 5,536,297: Marchbank et al) and regeneration techniques from RIL processes (U.S. Pat. No. 6,660,059: Ji et al) can be employed to reduce the reagent costs associated with thiosulfate leaching.
While thiosulfate on its own is considered environmentally benign, the presence of copper and ammonia in the leach, if ammonium thiosulfate is used, poses some environmental concerns. To mitigate the cost and environmental impact of ammonium thiosulfate leaching it is therefore advisable to regenerate and recycle the reagent. Sodium thiosulfate leaching has been proposed as an alternate; however, sodium thiosulfate is an expensive reagent.
The processes that have been disclosed to extract gold from the thiosulfate leach liquors include cementation on zinc (Berezowsky, et al.) or copper (Perez et al., Wan et al.), conversion of gold thiosulfate to gold cyanide, followed by adsorption on activated carbon (PCT Application WO 91/11539), and adsorption on ion exchange resin (Thomas, et al). Metallurgically these processes are very efficient but are not without limitations. For example, the cementation processes requires separation of the leach solution from the leach solids by filtration or counter-current decantation. Liquid solid separation processes incur high capital expense and gold losses from re-precipitation or entrainment in the leached solids may occur. The process disclosed in PCT Application WO 91/11539 also is also unsuitable for the treatment of carbonaceous preg robbing feed materials without solid/liquid separation prior to final gold recovery.
The process disclosed by Thomas et al., is used to recover gold thiosulfate from solutions or pulp without liquid-solid separation, and efficiently recovers gold from carbonaceous, preg robbing ores. Thiocyanate salts employed for gold elution, however are quite expensive, and thiocyanate losses to the tailings can have a negative impact on process economics and effluent management. In addition, the strong affinity of ion exchange resins for thiocyanate requires the use of sulfuric acid to displace thiocyanate and regenerate the resin. Regeneration with sulfuric acid is effective, but this process increases operating costs and reduces resin life. Strong base ion exchange resins used to recover gold from ammonium thiosulfate solutions also have an affinity for thiosulfate degradation products. Loading of polythionates on the resin will occur because the concentrations of polythionates are significantly higher than that of gold, leading to reduced gold loading, and a decrease in the number of cycles before resin regeneration is required.
Not all ores or other sources of gold are suitable candidates for gold extraction by direct leaching. Many precious metal deposits currently processed throughout the world are sulfidic in nature, and present certain challenges in the extraction and recovery of the contained gold. For example, gold contained in ores as very finely disseminated particles within a sulfide mineral crystal structure or as solid solution may be inaccessible to lixiviants. The cost of size reduction associated with liberating this gold is often prohibitive, and in the case of solid solution, ineffective. This problem has been overcome by oxidizing the sulfides contained in the ore, thereby liberating gold from the sulfide matrix and rendering it amenable to cyanidation. The methods of oxidation employed include bio-oxidation, roasting, atmospheric leaching, alkaline pressure oxidation as in the process disclosed in Mason et al., U.S. Pat. No. 4,552,589, or acidic pressure oxidation as disclosed in Thomas et al. U.S. Pat. No. 5,071,477, the entire disclosures of which are expressly incorporated by reference.
Acid pressure oxidation is typically performed by passing an ore or concentrate slurry through a multi-compartmented autoclave to which an oxygen-containing gas is continuously supplied. Pressure oxidation of sulfidic gold bearing ores is usually performed between 180° C. and 230° C. For certain ores, pre-treatment with sulfuric acid prior to pressure oxidation is required to neutralize the carbonates thereby maintaining acidic conditions in the autoclave and preventing oxygen losses when carbon dioxide is vented.
Some ores are characterized as double refractory because they are both sulfide refractory and preg robbing carbonaceous. In carbonaceous ores, preg robbing occurs when active carbon indigenous to the ore complexes the gold from cyanide leach solutions and reduces recovery. Pressure oxidation can partially deactivate the indigenous carbon, but is often insufficient for highly preg-robbing ores. In some instances, pressure oxidation has been shown to activate carbonaceous matter. An additional problem in recovering gold from highly carbonaceous ores is that a significant quantity of the gold may have been adsorbed onto carbon during formation of the mineral deposit. Cyanide has shown varying degrees of success in leaching gold locked in carbonaceous material, and in some cases, thiosulfate is recognized as a more suitable lixiviant. Several other approaches to reduce the impact of carbonaceous preg robbing have been employed with varying degrees of success. These include the addition of blanking agents such as kerosene or sodium lauryl sulfate or competitive adsorption during the carbon-in-leach process.